Hydrogenolysis of alkylbenzenes



United States Patent Ofiice 3,110,745 Patented Nov. 12, 1963 3,110,745 HYDROGENOLYSHS F ALKYLBENZENES I David W. Peck, Charleston, and Marion A. Eceles, Nitro,

W. Va., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Filed Dec. 1, 1960, Ser. No. 72,868 8 Claims. (Cl. 260-672) The present invention relates to the hydrogenolysis of substituted aromatic hydrocarbons, and more particularly, to an improved process for the hydrogenolytic dealkylation of alkylbenzenes.

The conventional vapor-phase dealkylation of alkylbenzenes by reaction with hydrogen at elevated temperatures to produce benzene and the corresponding alkane has, over recent years, received considerable attention from those skilled in the art. Such reactions, for example, find suitable utility and are of demonstrated interest in the production of benzene from the crude alkylbenzene-containing mixtures obtained by the aromatization of petroleum fractions or from coal tar or coal hydrogenation fractions. In this manner, convenient and economical processes for the production of benzene from commercially available raw materials are provided. Unfortunately, however, the yields of benzene that have been obtained via the hydrogenolytic dealkylation of alkylbenzenes have generally proven low. Consequently, more than occasional impetus has been given to the development of procedures for improving the production of benzene along these lines.

Heretofore, the hydrogenolytic dealkylation of alkylbenzenes has ordinarily been carried out by reacting hydrogen with an alkylbenzene at a temperature, and usually a pressure, sufliciently elevated so as to-engender the desired reaction. It has now been found that both the rate of dealkylation and the yield of benzene obtained by the hydrogenolysis of alkylbenzenes can be enhanced appreciably by the additional incorporation in the reaction mixture of an organic compound which, under the operating conditions utilized, as hereinbelow described, will decompose to form free alkyl radicals, and which will not otherwise react with the alkylbenzene in the reac-,

tion mixture so as to preclude the hydrogenolysis.

In its broadest aspect, the present invention contemplates employing as the alkylbenzene reactant a compound possessing as the alkyl grouping(s) thereof at least one alkyl radical preferably containing from 1 to about 3 carbon atoms. Typical of the alkylbenzenes found suitable in this respect there can be mentioned the following: toluene, xylene, ethylbenzene, propylbenzene, butylbenzene, ethyltoluene and the like. In addition, the use of a mixture of such alkylbenzenes as a reactant is also contemplated by the invention. Moreover, it is to be noted that when an alkylbenzene possessing more than one alkyl radical is employed as a reactant, more than one dealkylated product is often obtained; for example, both benzene and toluene are generally obtained by the hydrogenolytic dealkylation of xylene. Under such circumstances, it has been found that the yield of both dealkylated products is ordinarily enhanced by the process of this invention.

In the practice of the present invention, the hydrogenolysis of the alkylbenzene or alkylbenzene mixture is carried out in the vapor phase by heating a reaction mixture containing hydrogen, the alkylbenzene or alkylbenzene mixture, and the free alkyl radical-forming organic compound to a temperature of at least about 400 C. and preferably to a temperature of at least about 500 C. While somewhat lower reaction temperatures are also operable, an appreciable decrease in the rate of dealkylation and in the product yield is frequently encountered at such lower temperatures. On the other hand, the maximum reaction temperature which can be utilized for satisfactory operation is generally determined by the temperature at which undesirable side reactions occur, and can be ascertained readily by those skilled in the art in light of this disclosure. For instance, reaction temperatures of up to about 1000 C. and even higher can be employed efi'iciently. Within the operable reaction temperature range, an increase in the rate of dealkylation with increasing temperature has been observed. Byway of illustration, using a pressure of 2000 p.s.i.g., the residence time required for the substantial production of benzene is'of the order of hours when the temperature at which the reaction is carried out is in the range of from about 400 C. to, about 500 C.; the order of minutes when the reaction temperature is in the range of from about 500 C. to about 650 C., and the order of seconds at a reaction temperature above about 650 C. Thus, it can also be seen that the reaction time can be varied broadly in accordance with this invention.

Further, within the operable reaction temperature range hereinabove described, the reaction can be performed under either atmospheric or superatmospheric pressure. A variation in pressure, however, has not been found to have a considerable effect upon the reaction, although some slight increase in the rate of dealkylation has beennoted with increasing pressure. In operation, the maximum pressure which can be utilized is dependent primarily upon the pressure withstandable by the apparatus employed. Good results can be obtained, for'example, using pressures of up to 10,000 p.s.i.g. and higher. Ordinarily, the pressure as well as the residence time is decreased as the reaction temperature is increased. Thus, at a reaction temperature of 575 C., one might choose to operate at a pressure of about 3,000 p.s.i.g. with a residence time of about three minutes; while at a reaction temperature of 800 C., atmospheric pressure and a residence time of about 5 seconds might equally well be employed. In this connection, particularly eflicient results have-been obtained by carrying out the reaction at a temperature of from about 500 C. to about 640 C., under a pressure of from about 1000 p.s.i.g. to about 500 p.s.i.g.

Of prime importance to the process of this invention is the incorporation in the reaction mixture of a free alkyl radical fiorming organic compound that is to say, an organic compound which when heated to the reaction temperature employed, will decompose to form free alkyl radicals. More particularly, the free alkyl radical-formmg organic compounds which are preferably utilized in accordance with this invention are those which, under the reaction conditions hereinaboye described, decompose to form free alkyl radicals containing from 1 to about 3 carbon atoms, and more preferably, froml to 2 carbon atoms. As typical of compounds of this nature there can be mentioned the following, each of which possess at least one alkyl radical preferably containing tfroml to about 3 carbon atoms and more preferably from 1 to 2 carbon atoms: the alkyl ketones such as acetone, Z-butanone, 2- pentanone, 3-pentanone, butyrone, Z-hexanone, etc.; the alkyl ethers such as dimethyl ether, methylethyl ether, diethyl ether, dipropyl ether, methylbutyl ether, etc.; the alkyl aldehydes such as acetaldehyde, propionaldehyde, butyraldehyde, etc.; the lead tetraalkyls such as lead tetramethyl, lead tetraethyl, etc., and the like. Of these free alkyl radical-forming organic compounds, acetone, diethyl ether and acetaldehyde find especially preferred usage in the process of this invention.

The concentration in which the reactants can be utilized is not narrowly limited. For instance, the hydrogen and al-kylbenzene reactants can be employed in any proportion heretofore employed in the conventional hydrogenolytic dealkylation of alkylbenzenes and are preferably utilized in a molar ratio of from about 2 to about 10 moles of hydrogen per mole of alkylbenzene. Lesser proportions of hydrogen can also be used, accompanied, however, by a corresponding decrease in product yield. Similarly, higher proportions of hydrogen can be employed, although little commensurate advantage may accompany the use of a hydrogen to alkylbenzene ratio in excess of the preferred range.

The free alkyl radical-forming organic compound is usually incorporated in the reaction mixture in a weight proportion of from about 2 to about 50 parts of such compound per 100 parts of the alkylbenzene reactant, with the use of from about 5 to about 20 parts of the free alkyl radical-forming organic compound per 100 parts of the alkylbenzene reactant being preferred. In general, both the rate of dealkylation and the product yield have been found to be enhanced to an increasing extent as the proportion of the free alkyl radical-forming organic compound in the reaction mixture is increased within the broad range hereinabove described. Higher proportions of the free alkyl radical-forming organic compound can also be utilized, although little additional advantages may thereby be realized.

-In one embodiment of the present invention, the hydrogenolysis is carried out in a continuous process wherein a mixture of hydrogen, the alkylbenzene, and the free alkyl radical-forming organic com-pound is fed continuously to a suitable reactor such as a length or coil of stainless steel tubing maintained at the desired reaction temperature. The residence time is determined by the feed rate and the length of the reactor. The reaction products, along with any unreacted material, are then passed into a conventional, cooled liquid-gas separator. The gaseous components of the crude product, such as hydrogen, methane, etc., are removed from the top of the separator, While the liquid components, such as benzene, unreacted alkylbenzene, etc., are removed from the bottom 06: the separator, and can thereafter be subjected to further refinement. In addition to a continuous process, the process of the invention can also be carried out as a batch operation when desired, or in any other convenient manner.

The invention can be illustrated further in connection with the following specific examples of its practice, although it is not necessarily limited thereto.

EXAMPLE 1 In a series of experiments, a continuous teed containing gaseous hydrogen and liquid toluene was charged through a stainless steel cylindrical tube, 12 inches long by 0.5 inch inside diameter, having a volume of 40 cubic centimeters. The reactor was immersed in a lead bath and thereby maintained throughout at a uniform temperature of 575 C., at which temperature the toluene was vaporized. The reactants were introduced to the reactor in a hydrogen to toluene molar ratio of 4: 1, and the pressure within the reactor maintained at 2000 p.s.i.g. The rate of feed was controlled so that the residence time of the reactants within the reactor was two minutes. Upon emerging from the reactor, the eflluent crude was cooled in a liquid-gas separator and the liquid product collected. The experiment was also repeated in two additional runs utilizing as the liquid feed a mixture consisting of 5 grams of acetone per 100 grams of toluene, and in still another two runs, utilizing as the liquid feed a mixture consisting of 20 grams of diethyl ether per 100 grams of toluene. After each run, the collected products were analyzed by infra-red spectroscopy. The results obtained are tabulated below in Table A, wherein the amount of product recovered is indicated in percent by weight based upon the weight of toluene employed in the feed.

Table A Amount of Product Recovered Free Alkyl Radical-Forming Organic Compound Employed Benzene Toluene Total From the above table it can be seen that, other reaction conditions being constant, improved yields of benzene can be obtained by the hydrogenolytic dealkylation of alkylbenzenes wherein a free alkyl radical-forming organic compound is also incorporated in the reaction mixture.

EXAMPLE 2 Using the equipment described above in Example 1, a series of experiments Were conducted in which a continuous feed containing gaseous hydrogen and liquid toluene in a hydrogen to toluene molar ratio of 8:1 was heated and maintained at a' temperature of 600 C., at a pressure of 1000 p.s.i.g., for a residence period in the reactor of one minute. In one run, no free alkyl radical-forming organic compound was employed; in two other runs, the liquid feed consisted of 10 grams of acetone per 100 grams of toluene; and in still another run, the liquid feed consisting of 15 grams of acetaldehyde per 100 grams of toluene. Upon completion of the reactions, the liquid products were collected and analyzed as indicated above in Example l. The results obtained are tabulated below in Table B, wherein the amount of product recovered is indicated in percent by weight based upon the Weight of toluene employed in the feed.

Table B Amount 01 Product Recovered Free Alkyl Radical-Forming Organic Compound Employed Benzene Toluene Total EXAMPLE 3 Using the equipment described above in Example 1, a series of experiments were conducted in which a continuous feed containing gaseous hydrogen and liquid toluene in a hydrogen to toluene molar ratio of 9:1 was heated and maintained at a temperature of 585 C., at a pressure of 3000 p.s.i.g., for a residence period in the reactor of two minutes. Run No. 1 contained no free alkyl radical-forming organic compound; in run No. 2, the liquid feed consisted of 5 grams of acetone per grams of toluene; and in run No. 3, the liquid feed consisted of 33 grams of acetone per 100 grams of toluene. Upon completion of the reactions, the liquid products were collected and analyzed as indicated above in Example l. The results obtained are tabulated below in Table C, wherein the amount of product recovered is indicated in percent by weight based upon the weight of toluene employed in the feed.

From the above table it can be seen that, other reaction conditions being constant, by increasing the concentration of the free alkyl radical-forming organic compound within the proportions herein prescribed, a corresponding increase in the yield of benzene can be realized.

EXAMPLE 4 Using the equipment described above in Example 1, a series of experiments were conducted in which a continuous feed containing gaseous hydrogen and liquid toluene in a hydrogen to toluene molar ratio of :1 was heated and maintained at a temperature of 600 C., at a pressure of 2000 p.s.i.g., for a residence period in the reactor of two minutes. In one run, no free alkyl radicalforming organic compound was employed, and in another two runs, the liquid feed consisted of grams of acetone per 100 grams of toluene. Upon completion of the reactions, the liquid products were collected and analyzed as indicated above in Example 1. The results obtained are tabulated below in Table D, wherein the amount of product recovered is indicated in percent by weight based upon the weight of toluene employed in the feed.

Talble D Amount of Product Recovered Free Alkyl Radical-Forming Organic I Compound Employed Benzene Toluene Total None". so 59 so Acetone 49 42 91 Do 46 44 90 EXAMPLE 5 Using the equipment described above in Example 1, a series of experiments were conducted in which a continuous feed containing gaseous hydrogen and liquid toluene in a hydrogen to toluene molar ratio of 8:1 was heated and maintained at a temperature of 585 C., at a pressure of 3000 p.s.i.g., for a residence period in the reactor of two minutes. In one run, no free'alkyl radical-forming organic compound was employed; in another run, the liquid feed consisted of 11 grams of diethyl ether per 100 grams of toluene; and in a third run the liquid feed consisted of 11 grams of acetaldehyde per 100 grams of toluene. Upon completion of the reactions, the liquid products were collected and analyzed as indicated above in Example 1. The results obtained are tabulated below in Table E, wherein the amount of product recovered is indicated in percent by weight based upon the weight of toluene employed in the feed.

Table E Amount of Product Recovered Free Alkyl Radical-Forming Organic Free Alkyl Radical-Form- 0 EXAMPLE 6 Using the equipment described above in Example 1, a series of experiments were conducted in which a continuous fee-d containing gaseous hydrogen and liquid metaxylene in a hydrogen to meta-xylene molar ratio of 6:1 was heated and maintained at a temperature of 565 C., at a pressure of 2000 p .s.i.-g., for a residence period in the reactor of three minutes. In one run, no free alkyl radical forming organic compound was employed, and in two other runs, the liquid deed consisted of 20 grams of acetone per grams of meta-xylene. Upon completion of the reactions, the liquid products were collected and analyzed as indicated above in'Example 1. The results obtained are tabulated below in Table F, wherein the amount of product recovered is indicated in percent by weight based upon the weight of toluene employed in the feed.

Table F Amount of Product Recovered ing Organic Compound Employed Benzene Toluene Meta- Total Xylene None 4 22 66 92 Acetone 11 37 40 88 Do 10 36 45 91 From the table it can again be seen that, other reaction conditions being constant, improved yields OEf dealkylated products can be obtained by the hydrogenolytic dealkylation of alkylbenzenes wherein a free alkyl radical-forming organic compound is also incorporated in the reaction mixture.

The process or the invention is susceptible of further modification within the scope of the appended claims.

What is claimed is:

1. A process for the hydrogenolytic dealkylation of alkylbenzene which comprises heating to a temperature of at least about 400 C., a mixture containing hydro gen, at least one alkylbenzene and an organic compound capable of forming free alkyl radicals containing from 1 to 3 carbon atoms when heated to said temperature, said organic compound (a) being selected from the group consisting of the alkyl ketones, the alkyl ethers, the alkyl aldehydes and the lead tetraalkyls, (b) possessing at least one alkyl radical containing from 1 to 3 carbon atoms, and (c) being present in said mixture in a proportion of at least about 2 parts of said organic compound per 100 parts by weight of the alkylbenzene component of said 7 mixture.

taining from 1 to 3 carbon atoms, and (c) being present in said mixture in a'proportion of from about 2 to about 50 parts of said organic compound per 100 parts by weight of the alkylbenzene component of said mixture.

3. A process for the hydrogenolytic deallcylation of alkylbenzene which, comprises heating to a temperature of from about 500C. to about 650 C., under a pressure of firom about 1000 p.s.i.g. to about 5000 p.s.i.g., a mixture containing hydrogen, at least one alkylbenzene, said alkylbenzene possessing at least one alkyl radical containing from 1 to 2 carbon atoms, and an organic compound capable of forming free alkyl radicals containing from 1 to about 2 carbon atoms when heated to said temperature,

said organic compound (a) being selected from the group consisting of the alkyl ketones, the alkyl etbers, the alkyl aldehydes and the lead tetraalkyls, (b) possessing at least one alkyl radical containing from 1 to 2 carbon atoms,

and (c) being present in said mixture in a proportion of from about 5 to about 20 parts of said organic compound per 100 parts by weight of the alkylbenzene component of said mixture.

4. The process according to claim 3 wherein benzene is toluene.

5. The process according to claim 3 wherein said alkylbenzene is meta-xylene.

6. The process according to claim 3 wherein said organic compound is acetone.

said alkyl- 7. The process according to claim 3 wherein said organis compound is diethyl ether.

8. The process according to claim 3 wherein said organic compound is acetaldehyde.

References Cited in the file of this patent UNITED. STATES PATENTS Schneider Jan. 1, 1957 OTHER REFERENCES,

Noller: Chemistry of Organic Compounds, W. B. Sanders Co., Philadelphia (1951) (pages 543 and 544 relied upon). 

1. A PROCESS FOR THE HYDROGENOLYTIC DEALKYLATON OF ALKYLBENZENE WHICH COMPRISES HEATING TO A TEMPERATURE OF AT LEAST ABOUT 400*C., A MIXTURE CONTAINING HYDROGEN, AT LEAST ONE ALKYLBENZENE AND AN ORGANIC COMPOUND CAPABLE OF FORMING FREE ALKYL RADICALS CONTAINING FROM 1 TO 3 CARBON ATOMS WHEN HEATED TO SAID TEMPERATURE, SAID ORGANIC COMPOUND (A) BEING SELECTED FROM THE GROUP CONSISTING OF THE ALKYL KETONES, THE ALKYL ETHERS, THE ALKYL ALDEHYDES AND THE LEAD TETRAALKYLS, (B) POSSESSING AT LEAST ONE ALKYL RADICAL CONTAINING FROM 1 TO 3 CARBON ATOMS, AND (C) BEING PRESENT IN SAID MIXTURE IN A PROPORTION OF AT LEAST ABOUT 2 KPATS OF SAID ORGANIC COMPOUND PER 100 PARTS BY WEIGHT OF THE ALKYLBENZENE COMPONENT OF SAID MIXTURE. 